Most of my picks have already been mentioned
Toy Box - like someone else said above you really get the feeling that you’re in a toy store, I also loved how you could interact and manipulate items you normally wouldn’t even think of as being useful if you were just regular size going through the worlds.
The Caribbean - it was a big step up from KH II’s Port Royal, it looked beautiful and at times reminded me of Caribbean islands in real life, it was also fun to explore the different islands scattered across the world with each of them being uniquely designed, it really felt like you could explore it endlessly. I also liked the underwater exploration, and how there were hidden areas beneath, or that you needed to dive underwater to get to another part of the island you’re on.
Symphony of Sorcery - I’ve said it here before but fighting enemies to the sound of classical music tracks in the background is oddly satisfying. It was unexpected but a pleasant surprise.Each of the different environments in the world were so colorful too.
Hollow Bastion/Radiant Garden - it’s not that this world does anything super groundbreaking in terms of design, I just like that it’s one of the few that we get to see change throughout different points in time of the series, whether we get to see new areas of it that weren’t explored in previous entries or familiar areas that have just been altered based on what the world has been through.
The Realm of Darkness from 0.2 - again, nothing super groundbreaking, I just liked the atmosphere they created for it, seeing fragments of the worlds that had fallen to darkness with things being all messed up, out of place, and empty, along with the field theme that plays throughout really gave a sense of loneliness, despair, and endless wandering.